The present invention relates to fluid machinery for controlling a fluid pressure, and more particularly to a double screw rotor assembly, which uses pressure difference to adjust the clearance automatically, so as to reduce the consumption of starting power. The double screw rotor assembly of the invention can be used in vacuum pumps, air compressors, water or oil pumps, or other fluid media.
FIG. 1 shows a double screw rotor assembly manufactured by KASHIYAMA INDUSTRIES, LTD., and designed for use in a vacuum pump. This structure of double screw rotor comprises two screw rotors 81 and 82 meshed together. Because the screw rotors 81 and 82 have a constant pitch P' and constant height of tooth H', the volume of air chamber 810 or 820 does not change while air is transferred from the inlet to the output end 80, a significant pressure difference occurs and causes a reverse flow of air, high noises, and waste of energy.
U.S. Pat. No. 5,667,370 discloses another structure of double screw rotor assembly. According to this design, as illustrated in FIG. 2, the meshed screw rotors 83 and 84 have same height of tooth H", and the pitch is made gradually reduced in direction from the input side toward the output side 801 (P.sub.1 &gt;P.sub.2). Because of P.sub.1 &gt;P.sub.2, the volume of air chamber 830 or 840 is reduced during transmission, and the pressure in these chambers would be increased gradually. Therefore, when the air cambers were compressed and transmitted to the output end 801, less pressure difference occurs, the reverse flow of air would be reduced and so as to the noise. However, because of different pitches and pressure angles are defined at different rotor section, the fabrication process of the screw rotors 83 and 84 are complicated, resulting in a high manufacturing cost.
FIG. 3 shows still another structure of double screw rotor assembly, which was filed to USPTO for a patent by the present applicant under application Ser. No. 09/372,674. According to this design, two screw rotors are meshed together and mounted in a compression chamber inside a casing, each comprising a spiral thread around the periphery. The thread has a height H made gradually reduced from the input side to the output side 90. The threads of the screw rotors define a constant pitch P in order to be manufactured easily. The volumes of the air chambers 910 and 920 reduce gradually from the input side toward the output side, so the pressure can be increased gradually during transmission of air, the consumption of operation power and noise can be reduced. Because a uniform pitch P is provided and the height H is made gradually reduced from the input side toward the output side 90, the outer diameter D has the shape of an invertedly disposed cone, and the inner diameter d has the shape of a regular cone.
According to the aforesaid second and third prior art designs, much starting power is required when starting the double screw rotor assembly. As illustrated in FIG. 3, the pressure (i.e. the atmospheric pressure) in all air chambers 910 and 920, pressure Pi at the input side, and pressure Po at the output side, at the initial stage are the same. Because the volumes of the air chambers 910 and 920 are gradually reduced during rotary motion of the screw rotors, the pressure Pmax near the output side surpasses the pressure P0 (=the atmospheric pressure) at the output side when starting the double screw rotor assembly. Therefore, much more power and electric current are required to drive the rotors 91 and 92 to conquer the flow pressure of all air chambers 910 and 920. A certain period of time after starting, the flow pressure at the input side 901 is gradually reduced (for example, being drawn into a vacuum state), causing the flow pressure in the air chambers 910 and 920 near the input side 901 to be gradually reduced, and hence the power consumed is gradually reduced to the level of the rated working power. Because high working power is required when starting the double screw rotor assembly, high current, noise and vibration occur at the initial state when starting the screw rotors, resulting in an unstable operation.
FIG. 4 shows another prior art design constructed according to U.S. Pat. No. 5,533,887. According to this design, a movable case is sliding in a fixed case, however the spring at the top of the movable case is not adjustable, and the presence of the gap 22C which is left between the movable case and the fixed case for enabling the movable case to slide in the fixed case which may cause air leakage directly from the high pressure area to the low pressure area, thereby causing a low working efficiency. Further, if the process gas condensed in the gap between movable and fixed cases, the movable case may be jammed at some position, and the bypass mechanism failed.
In view of the drawbacks of the aforesaid prior art designs, there is a strong demand for a high performance double screw rotor assembly that requires low starting power, and can be conveniently adjusted to fit different manufacturing requirement.